The amount and distribution of coronary arterial narrowing by atherosclerotic plaque at necropsy is described in 70 victims aged 22-81 years (mean 50) of sudden coronary death (SCD). Of 3484 five-mm segments examined (mean 50 per patient) from the 4 major (left main [LM], left anterior descending [LAD], left circumflex [LC] and right) coronary arteries, 950 (27%) were narrowed 76-100% in cross-sectional area (XSA); 1127 (32%), 51-75%; 689 (20%), 26-50%, and 718 (21%), 0-25%. More extensive severe narrowing occurred in the proximal compared to the distal halves of the LAD, LC and right coronary arteries. Comparison between the 31 previously symptomatic victims (either angina pectoris and/or a clinical acute myocardial infarction [MI]) to the 39 victims who had previously been asymptomatic disclosed a significantly higher mean percent of severely (76-100% XSA) narrowed 5-mm segments (30%-vs-25% [p Less than .005]) and a lower mean percent of minimally (0-25% XSA) narrowed segments in the symptomatic group (15%-vs-25%, P Less than .001). Comparison of the 31 patients who had a healed MI at necropsy to the 39 patients who did not disclosed a higher mean percent of 5-mm segments narrowed 76-100% in XSA (33%-vs-24%, P Less than .001) and a lower mean percent of segments narrowed minimally in those with a left ventricular scar (13%-vs-26%, p Less than .001). Comparison between victims whose hearts weighed Greater than 450 gms to those weighing Less than or Equal to 450 gms disclosed a higher mean percent of severely narrowed segments (19%-vs-23%, P Less than .01) and a lower mean percent of minimally narrowed segments (29%-vs-24%, p Less than .005) in the large-heart group.